The present invention relates to CD4-independent variants of HIV, their proteins, and uses therefor.
HIV entry is known to require an interaction of the viral envelope glycoprotein (Env) with CD4 and cellular chemokine receptors. HIV Env protein is produced as a precursor (gp160) that is subsequently cleaved into two parts, gp120 which binds CD4 and chemokine receptors, and gp41 which is anchored in the viral membrane and mediates membrane fusion. Differential use of chemokine receptors by HIV and SIV has largely explained differences in tropism among different isolates (Berger, 1997, AIDS 11:S3-S16; Hoffman and Doms, 1998, AIDS 12:S17-S26). While a number of chemokine receptors can be utilized by HIV or SIV (Deng et al., 1997, Nature 388:296-300; Choe et al., 1996, Cell 85, 1135-1148; Rucker et al., 1997, J. Virol. 71:8999-9007; Edinger et al., 1997, Proc. Natl. Acad. Sci. USA 94:14742-14747; Liao et al., 1997, J. Exp. Med. 185:2015-2023; Farzan et al., 1997, J. Exp. Med. 186:405-411), CCR5 and CXCR4 appear to be the principal coreceptors for HIV-1 (Zhang et. al., 1998, J. Virol. 72:9337-9344; Zhang et al., 1998, J. Virol. 72:9337-9344). Isolates of HIV that first establish infection target blood lymphocytes and macrophages using CCR5 (Alkhatib et al., 1996, Science 272:1955-1958; Deng et al., 1996, Nature 381:661-666; Dragic et al., 1996, Nature 381:667-673; Doranz et al., 1996, Cell 85:1149-1158), while, viruses that are generally associated with progression to AIDS and can infect T cell lines in vitro use CXCR4 (Choe et al., 1996, Cell 85:1135-1148; Feng et al., 1996, Science 272:872-876; Connor et al., 1997, J. Exp. Med. 185:621-628).
Binding of Env to CD4 initiates poorly understood conformational changes enabling gp120 to bind to a chemokine receptor and leading to fusion of the viral and cellular membranes (Jones et al., 1998, J. Biol. Chem. 273:404-409; Moore et al., 1994, J. Virol. 68:469-484; Wyatt, 1992, J. Virol. 66:6997-7004; Wu et al., 1996, Nature 384:179-183). Immunologic and mutagenesis approaches have indicated that these changes involve movement of V1/V2 and V3 hypervariable loops on gp120 (Moore, et al., 1994, J. Virol. 68:469-484; Wyatt et al., 1992, J. Virol. 66:6997-7004; Wu et al., 1996, Nature 384:179-183), which play a critical role in the specificity of chemokine receptor utilization (Choe et al., 1996, Cell 85:1135-1148; Cocchi et al., 1996, Nature Med 2:1244-1247; Cho et al., 1998, J. Virol. 72:2509-2515; Speck et al., 1997, J. Virol. 71:7136-7139; Ross et al., 1998, Proc. Natl. Acad. Sci. U.S.A. 95:7682-7686; Hoffman et al., 1998, Proc. Natl. Acad. Sci. U.S.A. 95:11360-11365). The recent crystallographic resolution of a gp120 core structure bound to CD4 has revealed an intervening β sheet (the “bridging sheet”) between the inner and outer domains of gp120 that may serve as an additional contact site for the chemokine receptor (Wyatt and Sodroski, 1998, Science 280:1884-1888; Rizzuto et al., 1998, Science 280:1949-1953).
Although CD4 is generally required for gp120 to associate with a chemokine receptor, the identification of CD4-independent isolates of HIV-1, HIV-2, and SIV has demonstrated that functional interactions with chemokine receptors can occur in the absence of CD4 interaction (Edinger et al., 1997, Proc. Natl. Acad. Sci. USA 94:14742-14747; Reeves and Schulz, 1996, J. Virol. 71:1453-1465; Endres et al., 1996, Cell 87:745-756; Dumonceaux et al., 1998, J. Virol. 72:512-519). The determinants for the CD4-independent phenotype have been mapped to the viral env gene, but the underlying mechanisms of this phenotype are unknown. It has been proposed that mutations in env may increase the exposure and/or the affinity of the chemokine receptor binding site on gp120, thus circumventing the need for CD4 (Endres et al., 1996, Cell 87:745-756).
Biochemical assays have also shown that mutated or deglycosylated recombinant gp120 can bind directly to chemokine receptors, suggesting that domains normally activated by CD4 can be artificially exposed (Hesselgesser et al., 1997, Curr. Biol. 7: 112-121; Martin et al., 1997, Science 278:1470-1473; Bandres et al., 1998, J. Virol. 72:2500-2504; Misse et al., 1998, J. Virol. 72:7280-7288). A greater understanding of the determinants responsible for CD4-independence should provide insights into the Env domains that mediate and modulate interactions of Env with chemokine receptors and that ultimately govern viral entry.
To date, the ability of HIV-1 to escape the immune system has hindered development of efficacious vaccines to this important human pathogen. Thus, there is a long-felt and unfilled need for the development of effective vaccines and therapeutic modalities for HIV-1 infection in humans. The present invention meets those needs.